This PR adds Python 3.13 to the testing matrix.

It works out of the box on the Ubuntu builders, but it failed on Windows and MacOS because cffi failed to build.

There is currently a cffi rc release that is compatible with 3.13. When I added that all tests passed.
Once a final release is out, the explicit cffi dep can be removed. I added a comment as a reminder.

I would merge this now so that we can start testing on 3.13.
@dependabot should create a PR once a new version of cffi is out, reminding us to remove it.